Chromatin remodeling complexes can use energy of ATP hydrolysis to disrupt nucleosomes and regulate gene expression. BAF complex is a prototypical mammalian remodeler, containing ~10 subunits including the catalytic subunit Brg essential for remodeling. We found that deleting Brg in T cells impairs CD4 expression and immune tolerance. Surprisingly, a catalytically inactive Brg point mutant effectively rescues both defects, indicating Brg and hence BAF complex possess Remodeling Independent Function (RIF), which is consistent with the fact that six of 10 BAF subunits are dispensable for remodeling in vitro. The molecular mechanism of RIF is a mystery. To attack this problem, we will determine the effects of RIF on the physical structure of target genes, using the CD4 locus as a model (Aim 1). As a complementary approach, we will use Brg-deleted tumor cell lines to identify the BAF subunits and their functional domains mediating RIF, focusing on actin and the actin-related protein BAF53, a puzzling pair of subunits (Aim 2). Our study challenges a decade-old dogma about chromatin-modelers, and is of general interest because evidence is emerging that histone modifying enzymes also possess RIF-like activities and because the roles of nuclear actin have fascinated but largely eluded generations of biologists. PUBLIC HEALTH RELEVANCE: We found that an essential transcription-regulating enzyme harbors an unconventional activity that is unrelated to the classic catalytic function but nevertheless important for transcription regulation. Here we propose to address the molecular mechanisms of this novel activity, which is of medical implications given the biological importance of this protein.